This invention relates to a dual type manifold for use in the exhaust system of a multicylinder internal combustion engine which may be an automotive engine, the manifold being designed so as to enable installation of an exhaust gas sensor.
In recent automobiles, often an exhaust gas sensor such as an oxygen sensor is installed in an exhaust manifold to serve as an element of a system for feedback control of the air/fuel ratio in the engine. When the exhaust manifold is a so-called dual manifold, which is formed with two separate exhaust gas passages that are allocated respectively to two groups of engine cylinders into which all the cylinders are divided based on the firing order for the purpose of preventing loss of the engine output power by an interference of the pressure of exhaust gas discharged from any cylinder with the flow of exhaust gas discharged from another cylinder, it is not so easy to appropriately install an exhaust gas sensor in the manifold. In the dual manifold, the exhaust gas sensor must be installed so as to detect an average concentration of the specified component, e.g. oxygen, of the exhaust gases flowing in the two separate passages in the manifold, and therefore it becomes necessary to locally cut out a wall which partitions the two exhaust gas passages from each other to thereby expose the sensitive part of the installed sensor to both of the two exhaust gas flows in the respective passages.
However, the cutout of the partition wall in the manifold provides fluid communication between the two exhaust gas passages and, hence, is liable to cause an interference of the two exhaust gas flows with each other. Furthermore, there arises a possibility that the partition wall will suffer cracking in a region contiguous to the cutout as the wall undergoes repeated thermal expansion and shrinkage. Besides, when after-burning of the exhaust gases occurs in the exhaust manifold of this type the sensitive part of the exhaust gas sensor directly exposed to the exhaust gas flows in the two passages is liable to be damaged by the action of the exhaust gas pressure augmented by the after-burning. In conventional exhaust manifolds of dual manifold type, the above described problems relating to the installation of an exhaust gas sensor have not yet been solved completely. PG,4